Immune system has a skillful surveillance function to distinguish abnormal cells from autochtonous normal cells in the body and eliminate only the abnormal cells. When the surveillance function collapses, abnormal cells produced by mutation and the like cannot be eliminated, and the presence and growth thereof in the body are allowed. The grown abnormal cell mass is a tumor, or cancer.
The cancer treatment is mainly removal of cancer by surgical operation, or use of anti-cancer agents. However, these treatment methods often place physical burden due to extirpative surgery and side effects of anti-cancer agents, or mental burden due to operative scar on patients.
With such background, treatments by immunotherapy are drawing attention. In the immunotherapy, the number of the patients' own immunocytes is increased, and further activated to attack the cancer cells. As compared to surgical operation, physical burden on the patients due to the treatment is small, and an influence on the patients' social life due to the treatment can be minimized. In addition, a treatment method using immunotherapy and a surgical operation in combination is also employed. Since a tumor minimized by immunotherapy can be removed, the physical burden on patients can be reduced. Also, since the operative scar is small, the mental burden can also be drastically reduced.
Natural killer (hereinafter NK) T cells are immunocytes belonging to a new lymphocyte lineage that exhibit characteristics different from those of other lymphocyte lineages (T, B, and NK cells). NKT cells are related to NK cells because cytotoxic perforin granules are present therein (non-patent document 1). However, because NKT cells express not only NK cell markers, but also T cell receptors (TCRs), they have been shown to represent a new class of cells that are completely different (non-patent document 2). NKT cells can produce both Th1 type cytokine [mainly interferon (IFN)-γ] produced by T helper (Th)1 cell that promotes immunostimulatory action, and Th2 type cytokine [mainly interleukin (IL)-4] produced by Th2 cell that promotes immunosuppressive action (non-patent document 3). In other words, NKT cells can induce both activation and quieting of the immune system, which suggests the possible role of the immune system in the balance adjustment (non-patent document 4). Therefore, when the function of NKT cells can be controlled, various diseases, particularly cancer, caused by abnormal balance of the immune system can be treated.
The characteristic of NKT cells that is attracting the greatest attention resides in the fact that the α chain of TCR expressed in NKT cells is the same in all the individuals belonging to one certain species. This essentially shows that all NKT cells of the same species of organism are activated by recognizing the same substance. As such, the α chain is Vα24 for humans and Vα14 for mice, there is a very high homology between the two species. For the βchain, which forms a pair with the α chain, only a very limited number of kinds are known, and therefore, this TCR is called “invariant TCR”. It is also characteristic that TCR of NKT cells recognizes glycolipid, whereas TCR of T cells recognizes a protein fragment.
A wide variety of sphingoglycolipids are known to exist in living organisms. In general sphingoglycolipids in the living organisms, various sugars or sugar chains are bound to ceramides via β-bonds, and they are present in the cellular membranes of various organs.
Meanwhile, it is known that sphingoglycolipids comprising sugars bound to ceramides via α-bonds possess potent immunostimulatory action and antitumor activity. α-Galactosylceramides, typified by agelasphins, are glycolipids isolated from extracts from Agelas mauritianus, a kind of marine sponge, and have been reported to potently activate NKT cells (non-patent document 5). α-Galactosyl ceramides are taken by antigen presenting cells (APC) represented by dendritic cell (DC) and the like, and presented on a cellular membrane by CD1d protein similar to major histocompatibility complex (MHC) class I molecule. NKT cells are activated by recognizing a complex of the thus-presented CD1d protein and α-galactosylceramide by using TCR, whereby various immune reactions are initiated.
Heretofore, various analogs have been synthesized, and the correlation between the structure and the activity has been researched. It has been clarified that, among the series of synthetized analogs, KRN7000 (compound 1, α-GalCer) developed by Kirin Brewery Co., Ltd. shows an extremely strong anti-tumor activity, and the corresponding β-form (β-GalCer) does not show an immunostimulatory activity (patent document 1, non-patent document 6). KRN7000 is sphingoglycolipid comprising a ceramide resulting from the acylation of the sphingosine base by a long-chain fatty acid, and galactose bound thereto in α-configuration.

In recent years, with a focus on the above-described functions of NKT cells, a therapeutic drug of cancer, which contains KRN7000 as an active ingredient, has been developed. However, NKT cells activated by the administration of KRN7000 produce IFN-γ, which is a cytokine useful for the cancer treatment and inducing immunostimulatory activity, as well as simultaneously produce IL-4, which is a cytokine inducing an immunosuppressive action. As a result, the effects of the both are cancelled by each other, posing the problem of lack of sufficient effect of cancer treatment.
The group of Tsuji et al. has developed a glycolipid, α-C-GalCer, that strongly activates NKT cells of mouse and preferentially produces IFN-γ (compound 2, patent document 2, non-patent document 7). However, since α-C-GalCer scarcely induces cytokine production in human NKT cells, its clinical application is considered to be difficult. To solve this problem, a compound that shows a strong activity also in the human system has been developed in recent years (non-patent document 8).

A crystal structure analysis of human CD1d/KRN7000/TCR was reported in 2007 (non-patent document 9). According to the report, it has been clarified that the sugar moiety of KRN7000 is presented outside CD1d and towards TCR, whereas the ceramide moiety is stuck in a large hydrophobic pocket of CD1d. It was also found that oxygen atom on the pyran ring and the 6-position hydroxyl group of sugar of KRN7000 do not form a hydrogen bond with any amino acid residue of CD1d or TCR.
On the other hand, we have separately developed that novel synthetic glycolipid RCAI-56 (compound 3) having carbasugar and found that the compound strongly activates NKT cells and induces production of a large amount of IFN-γ (non-patent document 10). We have further developed that novel synthetic glycolipid RCAI-61 (compound 4) wherein the 6-hydroxyl group of the sugar moiety of glycolipid is modified, and found that the compound is more easily prepared than RCAI-56 and induces production of IFN-γ in large amounts (non-patent document 11). Since RCAI-56 and RCAI-61 show strong activity even in the systems of mouse and human (in vitro), its clinical application is expected.
However, since the synthesis of RCAI-56 requires multiple steps, and the synthesis of RCAI-61 requires complicated modification of sugar, the development of a novel analog permitting more convenient preparation and having an immunostimulatory activity equivalent to or higher than that of RCAI-56 and RCAI-61 has been desired. Furthermore, since RCAI-61 has a problem of low solubility in water, improvement of water solubility has been desired.

In 2006, Savage et al. developed PBS-57 (compound 5) which is a glycolipid wherein the 6-position hydroxyl group of the sugar moiety of KRN7000 is converted to an acetamide group, and the compound shows improved solubility in DMSO, as well as a stronger activity than KRN7000 (non-patent document 12). It has been reported in recent years that the compound shows a strong adjuvant activity (patent documents 3-5). Moreover, Calenbergh et al. developed a glycolipid (compound 6) wherein the 6-position hydroxyl group of the sugar moiety of KRN7000 is converted to benzamide (non-patent document 13). Since benzamide analogs have a weak IL-4 production induction activity, they induce relatively largely polarized IFN-γ production. In any report, however, synthesis of analogs wherein the 6-position is converted to an amide group requires a step of converting the 6-position hydroxyl group to an explosive azido group. Therefore, the safety becomes a problem for industrial-scale synthesis.

A number of analogs wherein acyl side chain is modified rather than the sugar moiety have been synthesized, and the activity thereof has been investigated. In 2010, Wong et al. reported the development of an analog having an aromatic ring on the acyl side chain (non-patent document 14). It has been reported that a glycolipid, 7DW8-5 (compound 7), having enhanced affinity of ligand and CD1d due to a π-π stacking interaction with an amino acid residue having an aromatic ring in the inside of hydrophobic pocket of CD1d, shows a stronger adjuvant activity than KRN7000.

Patent document 6 and non-patent document 15 disclose, as KRN7000 derivative, a synthetic glycolipid having carbamate at the 6-position.
Besides these, there are many reports on the analogs wherein the 6-position hydroxyl group is converted to an amide bond (patent documents 7-9, non-patent documents 16-22).